A commonly used photopolymer plate for letterpress printing is produced, for example, as described in Patent Documents 1 to 3, through an exposure step, a development step and a post-exposure step.
A printing method using a photopolymer plate for flexographic printing is a method in which an ink is supplied by using an ink supply roll or the like onto the surface of the raised areas of a polymer plate having raised and recessed areas and then, the polymer plate is brought into contact with a printing substrate so that the ink on the surface of the raised areas can be transferred onto the printing substrate. In such a flexographic printing, frequently in long-term printing, the ink sticks to the shoulders of the raised areas of the photopolymer plate, or gets into the recessed areas in the polymer plate (hereinafter, referred to as plate surface staining). Consequently, the portions other than the true images are sometimes printed erroneously. In such cases, it is necessary that printing is once halted, and the surface of the polymer plate is wiped away with a strip of cloth by using a washing liquid such as alcohol, which is economically disadvantageous.
On the other hand, the development of conventional photopolymer plates for letterpress printing generally adopts methods using organic solvents; however, from the viewpoint of environmental protection, plates permitting the use of aqueous developers are actively being developed. Techniques using such aqueous developers are proposed, for example, in Patent Documents 4 to 7 and the like. In water-developable photopolymer plates, there are used highly polar oligomers or monomers for liquid resins in order to impart water developability thereto. For solid resins, generally adopted is a technique in which a polar group-containing polymer is introduced as a polymer to constitute the main composition component, or a technique in which a polar group-containing polymer and a hydrophobic polymer are mixed and dispersed to be used. In particular, the latter method is preferably adopted because it leads to a better plate durability and permits forming a high-definition plate surface.
As a result of such composition characteristics, a water-developable photopolymer plate for letterpress printing is more increased in the polarity of the plate surface as compared to a solvent-developable photopolymer plate containing as the main component a hydrophobic polymer such as a thermoplastic elastomer, and hence is excessively increased in the affinity to the ink. Consequently, such an aforementioned phenomenon that the ink sticks to the shoulders of the raised areas, or gets into the raised areas in the polymer plate tends to occur more frequently.
In these years, the application of the letterpress printing to the high-definition printing has been promoted, and printing is made using halftone dots in order to develop the gradation in color and density. In such printing, excessive ink causes the ink fill-in around the relief of fine halftone dots used in a halftone printing with a large number of halftone lines, namely, the bridging between dots. Consequently, it causes marked degradation of printing quality to practically inhibiting the use of such halftone printing. Printing on plastic films has actively been carried out by using the letterpress printing. However, when printing substrates altered from the paper substrates to plastic film substrates low in ink absorbability, a conventional water-developable photopolymer plate for letterpress printing suffers from the following drawback: poor ink transferability results exclusively in low-density solid quality and in blurred printing; and attempt to compensate the poor ink transferability by increasing the printing pressure results in a slight increase in the solid printed portion density but, in return for it, in significant defects such as thick lines and dots, to inhibit a well-balanced printing.
Thus, conventional water-developable photopolymer plates for letterpress printing are limited in printable patterns and printing substrates because of the properties thereof in relation to the ink. Consequently, as affairs now stand, water-developable photopolymer plates for letterpress printing are limited in use in spite of the adaptability thereof to the environment.
Thus, as for the plate surface staining of a polymer plate, various methods have been proposed.
Patent Document 8 describes a technique in which an organic fluorine compound is adhered onto the surface of a plate by brush coating or spraying. However, this is effective only in halftone dot printing with a low number of lines and is insufficient in sustained effects, thus manifesting itself to be a method far away from solving the above described problems.
Patent Document 9 describes a method in which a material selected from a silane coupling agent, a silicone oil and a silicone grease is internally added in a photopolymer, and then such a material is made to migrate to the surface of the photopolymer. This method is a method aiming at prevention of generation of tear flaws in plate making operation and the like, and a method in which a silicone compound is basically made to bleed. Accordingly, the silicone compound is removed from the surface at the time of printing, so that the sustainability of the hydrophobic effect of the surface due to silicone cannot be expected at all.
Patent Document 10 proposes a method in which a mixture composed of an aqueous emulsion including a silicone compound or a fluorine compound and an aqueous resin is coated. However, the coating of an aqueous solution having a low penetrating ability does not necessarily provide a sufficient effect of preventing the plate surface staining. Such an effect can hardly be sustained particularly in a long run printing or a repeated printing, which necessitates repetition of such a coating in order to maintain the effect.
Patent Document 11 proposes a method in which the contact angle of the surface of the plate is made large by internally adding a hydrophobic compound, containing fluorine, chlorine or silicon, copolymerizable with the polymer material in a water-developable composition. This method is excellent in that such copolymerization with the photopolymer component makes it possible to fix the hydrophobilization of the plate surface to the resin. However, as mentioned in the specification, applicable resins are restricted to liquid or paste-like homogeneous resins such as polyurethane resins, polyvinyl alcohol resins, polyester resins or nylon resins. The present inventors have revealed that unexpected adverse effects may occur when such a technique is applied to a resin system, among water-developable photopolymers, in which a polar group-containing polymer and a hydrophobic polymer are mixed and dispersed, and which provides a plate with durability and allows forming a high-definition plate surface. The adverse effects are such that when a fluorine compound or a silicone compound is being added to the resin system in which a polar group-containing polymer and a hydrophobic polymer are mixed and dispersed, the fluorine compound or the silicone compound results in bleeding from the resin, or the plate becomes turbid to cause light scattering at the time of exposure, and consequently, image formation quality is degraded in such a way that there are caused, for example, failure in formation of fine lines and failure in shape of dots. Such addition of the hydrophobic compound as allowing suppressing to the utmost the degradation of image formation quality makes in turn the hydrophobic effect in the plate surface too small to attain the purpose. The operating mechanism to cause the turbid plate is not clear at present; however, it is inferred that the turbid plate may be ascribable to the light scattering amplified at the interface between the polar group-containing polymer and the hydrophobic polymer in which interface the hydrophobic substance tends to be present in the resin system wherein the polar group-containing polymer and the hydrophobic polymer are mixed and dispersed.    Patent Document 1: JP-A-10-171111    Patent Document 2: JP-A-63-088555    Patent Document 3: JP-A-05-134410    Patent Document 4: Japanese Patent No. 3508788    Patent Document 5: JP-B-58-33884    Patent Document 6: Japanese Patent No. 2940006    Patent Document 7: Japanese Patent No. 2985655    Patent Document 8: JP-A-51-40206    Patent Document 9: JP-A-60-191238    Patent Document 10: JP-A-2002-292985    Patent Document 11: JP-A-6-186740